1. Field of the Invention
The present invention relates to an electric power steering system and a method for controlling the electric power steering system.
2. Discussion of the Background
For safe driving in the rain, road surfaces are sometimes laterally sloped so that rain will not pool thereon. When a vehicle is running on such a laterally sloped road surface (hereinafter referred to as a cant road), it skids to a lower part of the road surface. Hereinafter, this phenomenon will be referred to as a “vehicle skid”.
To cause the vehicle to travel straight without skidding to the lower part of the road surface when such a vehicle skid occurs, the driver is forced to perform correction with the steering wheel. This increases the driving load on the driver.
Accordingly, a vehicle-skid preventing device for preventing a vehicle skid while reducing the driving load on the driver has been developed. For example, Japanese Unexamined Patent Application Publication No. 2007-168617 discloses an electric power steering system that corrects the steering assist amount so that the steering assist amount becomes larger when a vehicle skid is detected than when a vehicle skid does not occur.
One known control method for preventing a vehicle skid with the electric power steering system is to correct the steering assist amount in accordance with the steering torque detected by a steering-torque sensor when a vehicle skid occurs.
As the steering torque, however, the steering-torque sensor simultaneously detects both a torque applied to the steering wheel by the driver to prevent a vehicle skid and a torque applied to the steering wheel by the driver to turn the vehicle. Hence, if the steering assist amount is simply corrected in accordance with the steering torque detected by the steering-torque sensor, the amount of correction may become too large or too small.